In the manufacture of optical components and other precision components having precise tolerances for the component's geometry and surface qualities, the creation of thermal and material stresses must be minimized. The preferred abrasive tools are light in weight so as to permit high speed grinding while reducing stress on the grinding machine; have consistent wheel geometry and form holding ability; exhibit freeness of cut so as to minimize power draw and the accompanying stresses; minimize wheel loading; and are simple to dress, mount and otherwise handle during such operations.
High speed, light weight abrasive grinding wheels have been constructed from a variety of materials and typically comprise two parts: a hub and an abrasive rim. Solid or aluminum filled bronze or steel or solid or metal filled resin materials have been used in the core or hub component. In the abrasive rim component of the wheels, diamond or cubic boron nitride (CBN) abrasive grains are bonded in a matrix of metal or resin. Due to differences in the chemistries of the materials used in the core and the rim, respectively, and in their density and strength characteristics, together with differences in the functional purposes of the core and the abrasive rim components, the core and the rim components typically are constructed in separate operations. The abrasive rim component usually is constructed as a preformed module. Then the preformed module is joined to the rim of the core of hub with an adhesive cement, or by brazing, welding or similar techniques. Conventional processes are described in U.S. Pat. No. 4,378,233 and U.S. Pat. No. 3,925,035.
Manufacture of such tools is complex and costly. By pressing the lightweight core and the abrasive rim simultaneously, production costs are reduced. An example alternative process, which would be more labor intensive, consists of additional machining steps to fit the lightweight core and abrasive rim as well as mating the two parts with an adhesive or shrink fitting. Consistent wheel geometry and form holding ability are difficult to achieve in these processes for making light weight abrasive grinding wheels. Several improvements have been suggested, but none have addressed the manufacture of metal bonded abrasives on a light weight metal core in a satisfactory manner.
To attain the weight reduction that is critical to operation of these tools, cores have been made of bronze, molded to the final desired shape, and then hollowed out and filled with aluminum to lighten their weight. Different materials have been used in cores to attain operational considerations other than weight reduction. In U.S. Pat. No. 4,184,854 the wheels were designed to be mounted on a magnetic chuck, with optional magnetic holding parts, during the grinding operations. In making such wheels, the core is made of a resin filled with a magnetic metal powder (e.g., 43-72 wt. % iron) and aluminum powder, and the abrasive rim is a resin or a metal bond containing diamond abrasive grain. Zinc or tin may be substituted for the resin in the core to give an all metal bond. The tool is preferably constructed using the same resin in the core and rim so both components can be molded and cured simultaneously at a temperature of about 200.degree. to 300.degree. C. under sufficient pressure to achieve essentially theoretical density.
In GB-B-1,364,178 wheels are made by molding an aluminum powder core section and simultaneously sintering and bonding it to a polyimide resin diamond rim section at 350.degree. to 550.degree. C. by hot pressing.
In U.S. Pat. No. 4,042,347, a resin (polyimide) and metal powder mixture are co-sintered at a temperature of about 350.degree. C. to bond superabrasive grain in the rim component of a grinding wheel. The rim is bonded to a core of aluminum filled phenolic resin by an epoxy cement to make the finished wheel. The use of a core having the same co-sintered resin and metal powder mixture as the rim and substituting silicon carbide for superabrasive grain is suggested. This core would be joined to the rim by a cement.
In U.S. Pat. No. 5,471,970 saw blades for cutting concrete and other abrasive materials are made by molding metal powder bond components with abrasive grain around the perimeter of a preformed steel core and then sintering the molded tool at 760.degree.-1093.degree. C. (1400.degree.-2000.degree. F.) to achieve diffusion bonding of the abrasive rim to the steel core. In a second step, gullets are cut into the rim and, optionally, the perimeter of the core, to relieve stresses during cutting operations. Neither tool weight reduction nor continuous rim geometry are critical variables in making these saw blades.
Abrasive tools designed for chamfering operations on automobile windows and other glass substrates and having a metal bonded superabrasive grain rim on a resin core are described in JP-2-116475. The light weight of the resin core relative to conventional steel cores is taught to yield a 20-30% improvement in grinding time. The resin core is filled with powder of conductive metal and, optionally graphite powder, glass fiber or carbon fiber to allow electrical discharge machining of the wheels and to achieve core strength similar to that of steel cores. Attachment of the core to the rim is not described. An eccentric shaped rim, sandwich structures and concave/convex areas at contact points between the rim and the core are suggested as means to avoid detachment of the rim during grinding.
It has been discovered that abrasive tools having a metal core and a metal bonded abrasive rim may be made by sintering metal powder core and rim mixtures and joining the rim to the core in a single sintering step. By molding both components together during sintering, a near net shape tool is released from the mold. Higher porosity volume without loss of mechanical strength may be attained with this co-sintering process. The combined porosity of the rim and the core resulting from sintering the metal powders yields a light weight, mechanically strong tool capable of precision grinding operations at high speed.